Music that always plays on the radio and the TV, and is on the top 40 list on every music software like iTunes and Spotify. It is quick-made on a computer, usually featuring not very good but good looking musicians (if they can be called that) and it is un-original and boring. People without any taste in music listen to it because it's easy to find and everyone else listen to it, so then they don't need to worry about being considered "different". Mainstream music songs usually stay on the top-lists for a couple of months or maybe a year, and then they're gone. Also, people who listen to it usually would call a 2 year old song "very old", though bands like Led Zeppelin still rock anyone's brains out with their songs from the seventees.
Annoying teen girl: "Oh my god, have you heard that new Rihanna song? I love it!"
A month later, someone puts on the same mainstream music song
Same annoying chick: "Eww that song is old and outdated, turn it off"